Purdue University has identified an urgent need for a state-of-the-art multiphoton microscope by a number of NIH-funded investigators. Currently at Purdue University, there is no multiphoton excitation microscope available for general users including the NIH-funded investigators listed as major users. The lack of an available multiphoton microscope has been a major limiting factor for in situ imaging research of the investigators. Some key examples of the imaging needs are 1) trafficking of mucosal FoxP3+ regulatory T cells in the gut and associated lymphoid tissues;2) detection of circulating rare tumor cells in blood;3) interaction of platelets and immune/tissue cells in vivo;4) interaction of myeloid tumor suppressor cells with tumor cells in vivo;5) kinetics of adenoviral mediated gene transfer in vivo;6) host cell and Brucella interaction in vivo;7) the dynamic process of mammary epithelial cell development into the acinar structure in 3D culture;and 8) immune cell recruitment to and interaction with injected vaccines in vivo. All of these research projects are aimed to improve human health. We respectfully request for funding to acquire a state- of-the art multiphoton microscope. With the capability to keep emission fluorescence focused within the focal volume and use of an ultrafast infrared light for multiphoton excitation, the system has a number of desirable features for in situ imaging of live cells in 3D cultures and animals in a physiological setting. Purdue University is fully committed to support the acquisition of this equipment by providing space, resources, and FTE in tech and training support. The technical staff and investigators have the high-level technical expertise appropriate for effective use of the requested multiphoton microscope system. The multiphoton microscope system will be maintained in top shape by professional management through the Bindley Bioscience Center and the Purdue multiphoton advisory committee for effective and safe use by the NIH-funded investigators and others at Purdue. If funded, the multiphoton microscope will greatly improve the biomedical research environment at Purdue University and contribute to the advancement of human health.